1. Field of the Invention
This invention relates to a DC motor control circuit and a DC motor with the control circuit installed therein. More particularly, this invention relates to such motors suitable for use in toys of a vehicle.
2. Description of the Prior Art
Control circuits for unidirectionally driving DC motors according to JP-A-63-302780 and 4-109846 are applied by the present inventor. Other control circuits for bidirectionally driving DC motors according to JP-A-63-302779, 4-109846 and 4-112695 as well as EP-A-0 345 914 are also proposed by same inventor. Taiwan patent 179772 issued on Mar. 1, 1992 is equivalent to the combination of JP-A-4-109846 and 4-112695.
Referring now to FIG. 1, a control circuit for unidirectionally driving DC motor is shown. It comprises a drive transistor 11 capable of driving a DC motor 10. The drive transistor 11 is emitter coupled to a positive power supply line 13 connected to a positive electrode of a DC power supply 12. Its collector is connected to a brush 14 of the DC motor 10, and its base is connected to the collector of a detection transistor 15 to detect the rotation of the DC motor 10. The detection transistor 15 is emitter coupled through a resistor 16 to a ground line 17 connected to a negative electrode of the power supply 12. Also, another brush 18 of the DC motor 10 is connected to the ground line 17.
In the control circuit, transistors 11 and 15 are in OFF state under a static or rest condition where the DC motor 10 is not rotating. When the external force causes the rotation of a rotor of the DC motor 10, the DC motor 10 generates induced voltage pulses Vn each proportional to its rotation number N of the rotor and certain DC components between the brushes 14 and 18. EQU Vn=A.times.N
where, A is a constant. Each pulse has a pair of negative and positive pulse components when the rotating rotor of the motor 10 comes to a predetermined angular position relative to a stator thereof.
When the induced voltage Vn exceeds a certain threshold value, for example, 0.65 volt, the emitter current of the detection transistor 15 increases through the resistor 16, and then its collector current increases. Therefore, the drive transistor 11 whose base receives the collector current of the detection transistor 15 is turned on with its collector current being supplied to the DC motor 10 and the base of the transistor 15 to maintain the rotation of the DC motor 10. Also, the detection transistor 15 maintains its ON state.
In the rotating DC motor 10, impedance of the rotor coils is abruptly changed at angular positions between brushes 14 and 18. Succeeding narrow positive and negative pulse voltages called reactance voltages are periodically generated by an electromagnetic induction between brushes 14 and 18. With the negative pulses among them, the base-emitter voltage of the transistor 15 is reverse biased to cause the transistors 11 and 15 to instantly turn off. However, where the DC motor is rotating by an inertia, the drive transistor 11 will substantially maintain an ON state by the induced voltage of the positive DC component.
When the external force causes the rotating DC motor 10 to stop, the transistor 15 is turned off by the negative pulse irrespective that the constraint current generally continues to passing through the rotor coils. When the rotation number of the DC motor 10 is reduced to near the stop condition, the DC component of the induced voltage Vn between the brushes 14 and 18 is reduced under the threshold value, and then the detection transistor 15 is turned off after applying the negative pulse. Accordingly, the control circuit returns to the rest condition. The resistor 16 connected to the emitter of the detection transistor 15 acts as a negative feedback element to its collector current.
FIG. 2 shows another control circuit for bidirectional driving a DC motor. The control circuit comprises four drive transistors 21 to 24 bridge-connected between power supply terminals (circled 1 and 2), and a DC motor 10 connected between bridge points, i.e., output terminals (circled 3 and 4). The drive transistors 21 and 23 are emitter coupled to the positive power supply terminal (circled 1) or line 13 while the drive transistors 22 and 24 are emitter coupled to the negative power supply terminal (circled 2) or line 17.
The base of a normal detection transistor 25 for detecting an induced voltage upon the normal rotation is connected to a brush 14 of the DC motor 10 through a resistor 27. The emitter of a reverse detection transistor 26 for detecting an induced voltage upon the reverse rotation is connected to the brush 14 through the resistor 27. The emitter of the transistor 25 is connected to a brush 18 of the DC motor 10 through a resistor 28. The base of the transistor 26 is connected to the brush 18 through the resistor 28. The collectors of the detection transistors 25 and 26 are connected to the bases of the master drive transistors 21 and 23, respectively. Slave transistors 22 and 24 slave connected to the master drive transistor 21 and 23, respectively, have bases connected to output terminals (circled 3 and 4) through resistors 29 and 30, respectively.
When the normal detection transistor 25 is turned on by the external force applied to the DC motor 10, the drive transistors 21 and 22 are turned on to provide a current to the DC motor 10 from the terminals (circled 3 to 4), and then normal rotate the DC motor 10. Therefore, the transistors 25, 21 and 22 constitute a normal rotation driving circuit. Contrarily, when the reverse detection transistor 26 is turned on by the external force applied to the DC motor 10, the drive transistors 23 and 24 are turned on to provide another current to the DC motor 10 from the terminals (circled 4 to 3), and then reverse rotate the DC motor 10. Therefore, the transistors 26, 23 and 24 constitute a reverse rotation driving circuit.
Since the normal and reverse rotation circuits in FIG. 2 are symmetrical, the operation of the normal rotation driving circuit is discussed as follows. When the transistor 25 is turned on, its collector current is amplified by the master drive transistor 21. The current-amplified collector current of the drive transistor 21 is supplied to the brush 14 and the base of slave drive transistor 22 through the resistor 29. The collector of the slave transistor 22 sinks the current from the brush 18. A portion of the current supplied between brushes 14 and 18 of the DC motor 10 is supplied to the base-emitter of the detection transistor 25 through resistors 27 and 28 to maintain an ON state of the detection transistor 25.
The control circuit as shown in FIG. 2 kept the rest condition in a static condition of the DC motor 10. When the external force causes a rotor of the DC motor 10 to normal rotate, an induced voltage proportional to its rotation rate of the rotor is generated between the brushes 14 and 18. When this induced voltage exceeds a certain threshold value of the base-emitter voltage of the transistor 25, for example, 0.65 volt, the transistor 25 is turned on with increased base, emitter and collector currents through resistors 27 and 28. The collector current of the drive transistor 21 to which the collector current of the transistor 25 is provided as the base current increases and its collector current is partially supplied to the base of the slave drive transistor 22 through the resistor 29 to turn on the transistor 22.
While, the emitter current of the detection transistor 25 through the resistor 28 may be supplied to the base of the reverse drive transistor 24 to turn on it. However, since an ON resistance of the normal drive transistor 22 is lower than that of the reverse drive transistor 24, the transistor 24 transfers to OFF state. Therefore, the potential of the brush 14 of the DC motor 10 equals to the power supply voltage and the potential of the brush 18 equals to ground potential.
Reversely, when the external force causes the DC motor 10 to reverse rotate, transistor 26, 23 and 24 are sequentially turned on to maintain a reverse rotation condition after releasing the external force. The reverse rotating DC motor 10 also abruptly changed impedance of the rotor coils at angular positions between the brushes and generates successive narrow positive and negative pulses by the electromagnetic induction.between brushes 14 and 18.
When, for example, transistors 25, 21 and 22 are in an ON state and the DC motor 10 is rotating, the negative pulses generated between brushes 14 and 18 causes the base-emitter voltage of the transistor 15 to reverse bias and then these transistors to instantly turn off. However, where the DC motor is rotating, the control circuit maintains an active condition by the positive induced voltage.
When the external force restraints the rotating DC motor 10 to stop, the detection transistor 25 is turned off by the negative pulse irrespective that the constraint current generally continues to passing therethrough. When the rotation number of the DC motor 10 is reduced to near the stop condition, the DC component of the induced voltage between the brushes 14 and 18 is reduced under the threshold value, and then the detection transistor 25 is turned off. Accordingly, the control circuit returns to the rest condition.
The negative pulses generated between the brushes 14 and 18 of the DC motor 10 are applied between the base and emitter of the transistor 26. These pulses are absorbed by a capacitor 31 connected between the bases of the transistors 25 and 26. Accordingly, the transistors 26, 23 and 24 are not turned on.
As described the above, the control circuit shown in FIG. 2 can become two states, the active and rest states by the external forced rotation and stop of the DC motor as well as those of FIG. 1.
In such a DC motor control circuit, the restraint torque of the DC motor is changed depending upon the voltage change of the DC power supply which is generally a battery or dry cells. That is, higher power supply voltage causes an increasing restraint torque, while lower power supply voltage causes a decreasing restraint torque of the motor.
The conventional DC motor control circuit does not have a function for adjusting the constraint torque. In particular, when these control circuits are installed in electric toys and new dry cells are used as the DC power supply, it have a problem that the rotation of the DC motor would not stopped even if a predetermined load is applied to the DC motor because its power supply voltage is higher than that of the mean operation condition. Also, it is difficult to adjust a desired constraint torque.